Abstract
Controlled manipulation of particles in small-scale flows is a fundamental task in microfluidics, without which applications such as cell sorting, particle filtering, or droplet encapsulation would be impossible. When particles are not responsive to external forces (for example by carrying charges), one has to resort to hydrodynamic interactions in order to induce deviations of the particles from passively following streamlines. Even for small, spherical particles such interactions are not fully understood today. We highlight two themes of microparticle manipulation by hydrodynamic means: (1) The introduction of inertia via oscillatory flow of e.g. ultrasonic frequencies is a promising recent approach to increase the speed and throughput of particle deflection in microfluidics. We have derived first-principles effects of particle inertia that allow for controlled displacement of particles by size or density. The theory describes particle motion on oscillatory as well as oscillation-averaged time scales, thus predicting the steady displacements observed in experiments, and the results are in quantitative agreement with direct numerical simulations. (2) Even without inertia, i.e., in viscous Stokes flow, density-matched particles will be deflected from an initial streamline when encountering an obstacle or boundary. Well-defined changes in normal and tangential velocity components result from hydrodynamic interaction between particle and boundary. However, it has been underappreciated that such displacements need not be temporary if the geometry of the boundary or the geometry of the flow field break symmetry between the particle's approach and receding. We investigate the occurrence and magnitude of such net particle displacement both in external Stokes flow (around an obstacle) and in internal vortical Stokes flow. Of particular interest are scenarios where the particle is driven towards the interface so that the fluid layer between particle and boundary becomes extremely thin, so that in a practical application adhesion to the interface results. Comparing such effects of particle displacement at zero Reynolds number to those exploiting finite particle inertia may lead to new protocols of deterministic displacement or particle capture.
About the Speaker
Sascha Hilgenfeldt is a Professor in the Mechanical Science and Engineering Department at University of Illinois with a courtesy appointment in Physics. He joined U of I after a postdoctoral stay at Harvard and faculty appointments at University of Twente (Netherlands) and Northwestern University. He holds MS and PhD degrees in Physics from TU Munich and University of Marburg (Germany). His research covers experimental and theoretical aspects of fluid dynamics, soft matter, and the structure of biological tissues; He is a Willett Faculty Scholar and was elected a Fellow of the American Physical Society in 2016.
Host: Professor Leonardo Chamorro
